Composite bodies fabricated from mixtures of glass and crystalline particulates (ceramics) have been known commercially for a number of years, especially in the electronic packaging industry where glass has been commonly used as a flux to promote densification of alumina substrates. Because of the relatively low temperature softening points of glasses, research has been conducted over the years to devise thermally crystallizable glasses which, after sufficient fluxing to provide a bond for ceramic particles, would crystallize in situ to a glass-ceramic, thereby forming a composite body demonstrating higher refractoriness and greater mechanical strength than prior glass bonded bodies.
Numerous attempts to utilize glass-ceramics as bonds for particulate ceramics, most desirably alumina, have met with difficulties in achieving sufficient flow of the glass to provide strong bonding before crystallizing. Thus, prior to crystallizing, the glass must flow sufficiently to wet, encapsulate, and densify the crystalline ceramic particles.
Numerous potential applications for glass-ceramic-bonded ceramics have been investigated. For example, the oxidation of fibers (customarily resulting in embrittlement thereof) entrained within ceramic bodies might be reduced through the presence of the residual glassy phase following crystallization of the glass-ceramic bonding phase. Furthermore, glass-ceramic-bonded refractory phases, e.g., Al.sub.2 O.sub.3, ZrO.sub.2, Si.sub.3 N.sub.4, cordierite, etc., should permit lower sintering and hot pressing temperatures with increased mechanical strength and toughness, while still maintaining high use temperatures. Yet another possible application contemplates the inclusion of highly refractory particulate materials, such as Al.sub.2 O.sub.3 and/or ZrO.sub.2, as fillers in glassceramic coatings, which coatings should provide excellent barriers on metals or other substrates to attack by oxygen and hydrogen. Finally, a recent application has had the goal to extend the useful life of abrasive products, specifically grinding wheels and more especially grinding wheels wherein particulated Al.sub.2 O.sub.3 comprises the abrasive grain, by employing a glass-ceramic bond, instead of a glass bond, for the abrasive particles.
As was observed above, numerous attempts have been made in the past to utilize glass-ceramics as bonding materials for particulate ceramics. One example of that research is disclosed in U.S. Pat. No. 4,861,734 (MacDowell). Thus, that patent is directed to the manufacture of glass-ceramic bodies through the sintering of glass powders into an integral body accompanied with the development of crystals therein. The glasses were selected from the following composition intervals expressed in terms of weight percent on the oxide basis:
(a) 20-30% CaO, 35-55% Al.sub.2 O.sub.3, and 20-40% B.sub.2 O.sub.3 ; PA1 (b) 30-45% SrO, 30-45% Al.sub.2 O.sub.3, and 20-35% B.sub.2 O.sub.3 ; PA1 (c) 40-55% BaO, 25-40% Al.sub.2 O.sub.3, and 15-30% B.sub.2 O.sub.3 ; and PA1 (d) mixtures thereof.
In the preferred composition, the alkaline earth metal oxide, Al.sub.2 O.sub.3, and B.sub.2 O.sub.3 will be in an essentially 1:1:1 stoichiometry. The patent observed that the glass powders sintered into an integral body and crystallized in situ almost concurrently. That is, by the time the glass powders had sintered into an integral body, that body was substantially fully crystallized. Accordingly, whereas the glass powders described in the patent were useful in sealing applications, their viscosity characteristics did not recommend their utility in forming glass-ceramic-bonded ceramic composites.
Therefore, the principal objective of the instant invention was to develop glass compositions demonstrating physical properties, particularly viscosity characteristics, rendering them eminently suitable for forming glass-ceramic-bonded ceramic bodies and, more specifically, for forming glass-ceramic-bonded particulate Al.sub.2 O.sub.3 bodies.